The crystal structure and the band gap are important characterizations of semiconductor materials, both during and after growth. Crystal structure determination is particularly important for materials that can grow in more than one crystal form or in a mixed phase material. For example, although most III-V semiconductors, such as GaAs and InP, grow in the zincblende crystal structure with cubic symmetry, the III-V nitrides are important exceptions. For GaN, AlN, InN and alloys between these compounds the lowest energy configuration is in the wurtzite or hexagonal crystal structure. When these nitrides are grown on a substrate of a different material, the resulting film can be a mixture of wurtzite and zincblende regions. In production of III-V nitride devices such as LEDs, transistors and lasers, characterization of the crystal structure is required.
X-ray analysis can be used to determine the relative amounts of different crystal structures in a mixed-phase specimen (Abernathy et al., Appl. Phys. Lett. 66, 1995, p. 1632). However an x-ray system is expensive and cannot readily be used during crystal growth. Reflection high energy electron diffraction (RHEED) can be used to determine the crystal phase during crystal growth (Okumura et al., Appl. Phys. Lett. 59, 1991, p. 1058). However, RHEED is difficult to quantify in terms of volume fraction and neither RHEED nor x-ray analysis gives band gap information. RHEED is also limited to growth techniques taking place under vacuum conditions.
Reflectance difference spectroscopy (RDS) has been used to study the surface structure of purely zincblende III-V semiconductors during crystal growth (Aspnes et al., J. Vac. Sci. Technol. A 6, 1988, p. 1327). The cancellation of the bulk signal due to the crystal symmetry makes it possible to measure the much smaller contributions of the surface to the reflectivity. RDS has also been used to study bulk ordering in zincblende alloys (particularly Ga.sub.0.52 In.sub.0.48 P). With bulk ordering, the crystalline phase remains unchanged but the species rearrange within the layers. The partial bulk ordering breaks the zincblende symmetry of the lattice and thereby produces an RDS signal. Polarized reflectance spectroscopy has also been used to obtain the refractive indices of hexagonal GaN (Yu et al. (1997) Jpn. J. Appl. Phys. 36(8A):L1029).